1. Field of the Invention
The present invention relates to a hydraulic shock absorber for use in a car or the like for adjusting a damping force according to a road condition at the time of driving.
2. Description of the Prior Art
Effective adjustment of a damping force for optimum comfort and steering stability according to road conditions has been proposed hitherto for hydraulic shock absorbers used in cars or the like.
There has been proposed, for example, a hydraulic shock absorber (as disclosed in Japanese Utility Model Unexamined Publication No. 61-1729, for example) wherein a bypass is formed within a piston rod through which two chambers in a cylinder partitioned by a piston communicate with each other, and a plurality of orifices are provided halfway along the bypass. The orifices are opened and closed by an externally driven shutter, whereby damping force characteristics are thus modified.
Also proposed is a hydraulic shock absorber (as disclosed in Japanese Utility Model Examined Publication No. 59-22359, Japanese Utility Model Examined Publication No. 60-37477, for example) wherein two partitioned chambers are kept in communication through a plurality of passages, a damping force generating mechanism (such as an orifice or the like) is provided for generating a damping force by controlling a flow of hydraulic fluid generated in the passages during a sliding motion of a piston in a cylinder, and the damping force is adjusted by opening and closing the passages by the use of a solenoid valve.
In a hydraulic shock absorber for adjusting a damping force by the selective operation of a plurality of orifices according to the operation of a shutter, more than two damping forces can be effectively selected and therefore, a high degree of control is realizable in response to road conditions and other factors. However, in the case of adjusting a damping force by the operation of a solenoid valve, only two damping forces may be selected and therefore, a damping force precisely corresponding to road conditions and other factors is not obtainable.